1. Field of the Invention
The present invention relates to a surface-mount casing for housing therein a quartz crystal blank to construct a surface-mounted quartz crystal unit, and more particularly to a surface-mount casing which is capable of selectively housing therein crystal blanks having different sizes and a crystal unit using such a surface-mount casing.
2. Description of the Related Art
Surface-mount crystal units which are surface-mounted on wiring boards are small in size and weight, and hence are incorporated especially in oscillators in portable electronic devices. The Surface-mount crystal units generally employ a surface-mount casing comprising a casing body which has a recess defined in one principal surface thereof. A crystal blank is fixedly mounted on the bottom of the recess in the casing body, after which the opening of the recess is closed by a cover, hermetically sealing the crystal blank in the recess. Generally, the crystal blank is fixed to the casing body at two points on its outer circumferential portion. With crystal units for use in mobile devices such as cellular phone units, the crystal blank is generally fixed in position at opposite sides of one end thereof. With crystal units for use in other electronic devices, the crystal blank is generally fixed in position at opposite longitudinal ends thereof. There is also known a crystal unit having a substantially rectangular crystal blank that is fixed to a casing body at two positions on respective opposite ends of one diagonal line of the crystal blank.
FIGS. 1A and 1B shows a conventional surface-mount crystal unit by way of example. The crystal unit has casing body 1 in the shape of a substantially rectangular parallelepiped with a recess defined therein, crystal blank 2 disposed in the recess, and cover 3 mounted on casing body 1 to hermetically seal crystal blank 2 in casing body 1. Connecting terminals 4 for electrically connecting to crystal blank 2 are disposed on the bottom of the recess at respective positions on opposite ends of one diagonal line of the bottom of the recess. Mounting electrodes 5 are disposed on an outer bottom surface of casing body 1 for mounting the surface-mount crystal unit on a wiring board. Connecting terminals 4 and mounting terminals 5 are electrically connected to each other by electrically conductive paths (not shown) such as through holes in casing body 1. Mounting terminals 5 may be disposed respectively in central regions on a pair of opposite sides of the bottom of casing body 1 or may be disposed respectively on four corners of the bottom of casing body 1.
FIG. 2 shows a structure of crystal blank 2 by way of example. Crystal blank 2 comprises a substantially rectangular AT-cut quartz crystal blank, for example. Excitation electrodes 6 are disposed coextensively on the respective opposite principal surfaces of crystal blank 2. Extension electrode 7 extends from one of excitation electrodes 6 to a corner of crystal blank 2 along one diagonal line thereof, and another extension electrode 7 extends from other excitation electrode 6 to an opposite corner of crystal blank 2 along the same diagonal line thereof. Extension electrodes 7 are folded back a short distance over the other principal surfaces across the diagonal corners. The diagonal corners to which extension electrodes 7 extend are bonded to respective connecting terminals 4 of casing body 1 by electrically conductive adhesive 8, thus electrically and mechanically connecting crystal blank 2 to casing body 1.
Since crystal blank 2 is held in position at the opposite ends of one diagonal line, if the longitudinal direction of crystal blank 2 is aligned with Z′ axis, in particular, of the crystallographic axes (X-, Y′-, and Z′-axes) of quartz crystal in the AT-cut quartz crystal blank, then the direction in which crystal blank 2 is held approaches the Z-axis ±30° directions where the stress sensitivity of the quartz crystal shows null. Therefore, the crystal unit exhibits good stress sensitivity characteristics such that adverse effects imposed on frequency vs. temperature characteristics by stresses applied to crystal blank 2 due to temperature changes are reduced.
However, since the above surface-mount crystal unit uses crystal blanks having different sizes, its outer profile differs depending on the vibration frequency, for example. The surface-mount casing has its outer profile dimensions determined as de facto standards, i.e., 8 mm×4.5 mm. Therefore, if crystal blanks having different outer profile dimensions, particularly longitudinal dimensions, are housed in surface-mount casings having the same outer profile dimensions, then it is necessary to change the distance between connecting terminals 4 on the bottom of casing body 1.
If crystal blank 2 has a larger outer profile dimension or length, then connecting terminals 4 need to be formed shorter, and if crystal blank 2 has a smaller outer profile dimension or length, then connecting terminals 4 need to be formed longer. For this reason, it is necessary to keep in stock a plurality of casing bodies 1 with connecting terminals 4 having different lengths, with the result that production management schemes become complex and the cost of the surface-mount crystal unit is high. Though the AT-cut quartz crystal blank has its vibration frequency determined depending on its thickness, its optimum outer planar dimensions are determined in view of vibration characteristics and shock-resistance capability. Generally, as the vibration frequency is higher, the outer dimensions of the crystal blank are smaller.